At least 15 people have been killed in Borno state when four female suicide bombers attacked four areas in Maiduguri.

Suicide bombers have killed at least 15 people and injured 21 in the northeast Nigerian city of Maiduguri, police officials confirmed.

Tuesday night incident is the latest in a spate of suicide bomb attacks on the city in the last few weeks. Borno, of which Maiduguri is the capital, is the Nigerian state worst affected by the eight-year-old fight by the Boko Haram group.

Borno state police commissioner Damian Chukwu told reporters that most of the victims were civilian group manning security posts.

"The bombers detonated IEDs (improvised explosive devices) strapped to their bodies at different locations of the area, killing 19 people, including the bombers," he said.

"A total of 23 people were injured."

Bello Danbatta, a spokesman for the Civilian Joint Task Force (JTF) group and chief security officer at the Borno State Emergency Management Agency (SEMA), said it appeared his men were the targets.

Two of the bombers blew themselves up at checkpoints manned by JTF members, who assist the military with security and sometimes accompany soldiers on operations against Boko Haram jihadists.

"In all we lost 12 of our gallant JTF," he said.

Waiting for burial

SEMA operatives in face-masks and white overalls were on Wednesday seen removing body parts from the scene of the attacks. Victims were covered with rugs awaiting burial, as local people looked on.

At least 20,000 people have been killed and more than 2.6 million made homeless in northeast Nigeria since the start of Boko Haram's armed campaign in 2009.

Nigeria's government and military maintain that Boko Haram is a spent force, but intermittent attacks and suicide bombings pose a constant threat, particularly in remote areas.

Young women and girls have frequently been used to attack security checkpoints, as well as civilian "soft targets" such as mosques, markets and bus stations.

Nine people were killed in a string of suicide bomb attacks in the city last month around the Eid al-Fitr holiday marking the end of the Muslim holy month of Ramadan.

The University of Maiduguri, which lies on the edge of the city, has become a frequent target since the start of the year, as it teaches the "western" education despised by the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) affiliate, Boko Haram.

Boko Haram this week released a video showing executions and amputations, suggesting it still hold territory in some areas.